Use QStringRef in parsing plugin-specs.

Avoid unnecessary string allocations, fix error reporting
which would pass PluginSpec::name instead of element to the
error message.

Task-number: QTCREATORBUG-9002
Change-Id: I0d136ef27f4226e1ba79e74ddab6b63cf0f20c74
Reviewed-by: hjk <hjk121@nokiamail.com>